1. Field of the Invention
The present invention relates to a design for core liquid retention for liquid core light guides and more particularly to a design for core liquid retention during temperature cycling between 0.degree. F. and 140.degree. F. for liquid core light guides.
2. Related Prior Art
In the use of liquid core light guides problems exist in temperature induced volumetric variations. Typically, the liquids that are used significantly change volume as a function of temperature, specifically the range of 0.degree. F. (-18.degree. C.) to 140.degree. F. (60.degree. C.). During the heating of a LLG (Liquid-core Light Guide) containing such a fluid, the pressure created by volumetric expansion of the liquid is sufficient to slightly deform (i.e., elongate) the Teflon.TM. tubing. This deformation is permanent, resulting in the formation of a small bubble when the liquid core light guide is returned to normal room temperature and the liquid undergoes volumetric contraction. This effect is rooted in the significant difference between the larger volumetric expansion coefficient of the liquid as compared to that of the FEP (Teflon.TM.) tubing. The resultant bubble is large enough to drastically reduce the delivered light intensity from the liquid core light guide.
In a present commercial liquid core light guides, most commonly Series 300 from Lumatec of Munich Germany, the core liquid has a volumetric expansion coefficient that closely matches that of the Teflon.TM. tubing.
Several liquids are available that can be used to make a liquid core light guide transmit efficiently over a range from 350 to 800 nm., namely Dupont Syltherm XLT, General Electric SF-96-50, and Fluka UV-grade mineral oil. The first two are silicone oils (poly-dimethyl-siloxanes) and have been found to be radiation-hard under a 1,000 hour irradiation from ultra-violet (UV) light (centered at 365 nm) that varied in intensity from 1-4 W/cm.sup.2. The third is not radiation-hard, but is a very efficient transmitter of visible and near-IR light. All three are relatively inexpensive. However, none of these possible liquids have a volumetric expansion coefficient that closely matches that of the Teflon.TM. tubing and were undesirable for operational use throughout the aforementioned temperature range because of the deleterious effects due to this difference.
In addition, three new high index of refraction fluids (1.49-1.58) have been identified that can be used for efficient visible light transmission, and moreover, these fluids can used as low toxicity, non-flammable liquid scintillator bases. These are first, phenyl-xylyl-ethane (PXE), second, linear alkylbenzene (LAB) and third, isopropyl-biphenyl (IPB). With the addition of typical scintillator fluors such as PTP, BPBD, POPOP, bis-MSB, TPB, BBQ, Y7 and 3HF, one can create an active scintillating medium for use in Nuclear Science applications requiring the flexibility afforded by the liquid core light guide. In the case of PXE and LAB, such liquid scintillators can be obtained from Fisher Scientific under the Scintisafe.TM. trademark.
When compared to a commercial reference standard liquid core light guide such as series 300 from Lumatec of Germany, a UV-grade liquid core light guide can deliver up to 85% of the UV intensity (365 nm) of the Lumatec liquid core light guide at a much lower cost. However, the liquid core light guide cannot withstand a temperature cycling test such as the following. First, from room temperature, cool the liquid core light guide to 0.degree. F. for 72 hours, and then return to room temperature. Second, proceed to heat the liquid core light guide to 1400.degree. F. for 72 hours, and then return to room temperature.
During the cooling cycle, the liquid is seen to contract leaving a vacuum air bubble. Upon reheating (slowly) to room temperature, the bubble disappears and the liquid core light guide operates normally. However, after the light guide is heated and then allowed to cool as in the second step, one sees a bubble developing even though no leakage of fluid is detected. Studies of the volumetric expansion coefficient and measurements of the pressure developed during the heating of the light guide suggest strongly that the FEP Teflon.TM. is permanently deformed during the heating. This apparent increase in the tubing volume results in a slight underfilling of the tubing after the cooling to room temperature.
If the light guide can be used in normal room temperatures, for example from 600.degree. to 80.degree. F., then the normal design of the light guide can be used. For cases where the light guide operates in normal temperature conditions, but may be exposed to extreme temperatures for a short time, such as during shipping, or in cases where the light guide may have to operate in abnormal temperature conditions, an alternate design is required.
The following patents are indicative of the state of the art addressing this problem of bubble formation during the cooling process after heating.
U.S. Pat. No. 5,33,227, titled "Optical Waveguide Hose", issued to Minoru Ishiharada et al., relates to an optical waveguide hose including a hollow tubular cladding, a fluid core in the cladding, the fluid having a higher index of refraction than the cladding, and sealing plugs mated with opposite end opening of the cladding. In one form, the core fluid is filled in the cladding under positive pressure. In another form, a sheath of gas barrier material encloses the outer periphery of the cladding. The pressurized fluid core of the gas barrier sheath prevents air from penetrating into the core, allowing the hose to maintain its light transmission function.
International Patent Application, PCT publication number WO 95/121138, published on May 4, 1995, titled "Liquid Core Optical Waveguide", applied for by Frederick Harold Eastgate, relates to an optical waveguide for transmitting radiation. In one embodiment the waveguide includes a flexible tube of a material that is substantially transparent to the radiation, and a liquid core filling the tube having a refractive index greater than that of the tube material. In another embodiment the waveguide includes a tube having an inner lining, and a liquid core filling the tube and having a refractive index greater than that of the lining material. In a still further embodiment there is an optical waveguide for transmitting radiation and functioning as a non-imaging concentrator. The waveguide includes a tube having an input end and/or an output end that is greater than the diameter of the tube body.
U.S. Pat. No. 3,995,934, titled "Flexible Light Guide", issued to Gunther Nath, relates to a flexible light guide of the liquid filled type with a liquid supply container outside the light guide. In accordance with one form of the device described, a flexible light guide has a column, operating as an optic fiber or a liquid, which absorbs as little as possible of the wavelength range to be transmitted, with a predetermined index of refraction. This column is surrounded by a flexible tube of plastics material, which, in the wavelength range to be transmitted, has a somewhat lesser index of refraction than the liquid. In accordance with the device described, such a light guide has a supply container filled at least partly with the liquid and connected with the interior of the flexible tube.
The presence of the supply container ensures that the flexible tube is always completely filled with the light conducting liquid, even if in the course of time liquid should be lost from the tube. The supply container furthermore ensures satisfactory filling of the flexible tube and high transmission of the light guide even on bending of the flexible tube and in the case of high thermal loading of the liquid column owing to high intensities of radiation.
The foregoing methods and apparatus, although each effective for its limited purpose, do not solve the problem presented of permanent deformation of the Teflon.TM. tubing caused by the pressure created by the volumetric expansion of the liquid during the heating of a liquid-core light guide, resulting in the formation of a small bubble when the light guide is returned to normal room temperature.